The present invention relates to nerve monitoring, and more particularly, to a device to facilitate nerve monitoring.
A risk presented by thyroid surgery, parathyroid surgery, skull base surgery, cervical spine, or any other surgery in the space around the oropharynx, larynx, trachea or esophagus, is damage to the Recurrent Laryngeal Nerves (“RLN”). RLNs control the vocal cords, and damage to them can result in full or partial vocal cord paralysis. An issue with RLNs is that they are small and difficult to identify, particularly where surrounding tissue is bloodied, inflamed or otherwise disrupted due to surgery or trauma. Another issue is that simply trying to identify RLNs by touch can stretch or tear those nerves, which can result in hoarseness, difficulty in speech, aspiration of food or liquids (which can result in pneumonia), and life-threatening airway obstruction.
Accordingly, there have been recent efforts to use intraoperative RLN monitoring techniques, with the objective of reducing the risk of damage to the RLNs and subsequent vocal cord impairment or paralysis. One advocated form of RLN monitoring implements electromyography (EMG) to protect the nerves.
A common procedure in which laryngeal EMG is used is a thyroid surgery. In this procedure, a specialized endotracheal tube (ET tube) is placed through the patient's nose or mouth and into the trachea to assist in respiratory ventilation and/or to provide anesthesia. The ET tube also passes between the sets of laryngeal muscles, and typically rests adjacent the left and right posterior cricoarytenoid muscles. The specialized ET tube includes a pair of exposed, cylindrical wires on its external surface or embedded therein. These wires form electrodes that are intended to contact the various vocal muscles when the ET tube is (a) properly inserted at the correct depth, and (b) properly rotationally oriented relative to the trachea and larynx. These electrodes of the ET tube are capable of detecting EMG signals generated by an electrical probe. An example of such a specialized tube is disclosed in U.S. Pat. No. 5,125,406 to Goldstone, which is hereby incorporated by reference.
During the procedure, a surgeon applies the electrical probe to the area in which he believes the RLN is located. If the electrical probe applies voltage to or near the RLN, the electrical pulse is carried to the vocal muscles (primarily the “thyroarytenoid muscles” along the vocal cords anteriorly and the “posterior cricoarytenoid muscles” posteriorly) through the RLN, which in turn causes contraction of the vocal muscles which generate their own electric pulse. The respective wire electrode on the ET tube facing the stimulated vocal muscles subsequently detects the electromyographic (EMG) response. The detecting electrode transfers a signal to a receiver or EMG monitor, which emits an audio or visual alarm. This output alerts the surgeon that the probe is close to the RLN so that the surgeon can confirm the nerve's location and minimize trauma in the probed location.
One commercially available instrument suitable for the above procedure is the Kartush Stimulating Dissection Instruments (KSD), which allow ongoing electrical mapping of the nerve's location during surgical dissection by simultaneous stimulation and surgical dissection. Education, however, is required of thyroid and other surgeons using the above procedure to assure appropriate Stimulating Dissection to minimize false positive and false negative stimulation errors.
Another challenge concerning the above procedure concerns minimizing false negative and positive recording errors, especially related to contact between the electrodes on the ET tube and the laryngeal muscles to monitor the RLN. It is frequently difficult to ensure adequate Electrode-Vocal Cord (EVC) contact both as the ET tube is being inserted in the patient and after the ET tube is positioned. In other words, the ET tube electrodes used to monitor the RLN can be difficult to accurately place, as well as difficult to maintain in proper position.
Obtaining sufficient EVC contact is limited by several factors. First, direct visualization of the EVC juxtaposition typically occurs only during intubation. Even if the ET tube is checked immediately after positioning in the patient, loss of appropriate EVC contact may go undetected if it is not repeatedly checked. Further, the anterior location of the larynx or a large, floppy epiglottis can prevent direct visualization of EVC contact, even with a laryngoscope. Although this can be overcome by a flexible scope, the time and expense to add intermittent or ongoing flexible fiber optic endoscopy following standard intubation with a rigid laryngoscope can make this procedure impractical.
Second, the electrodes of the current and previous devices are positioned on a round ET tube, however, the aperture of the human glottis, i.e., the glottic opening, is triangular. This creates a fundamental mismatch between the geometry of the ET tube and the laryngeal surfaces, such as the glottic opening and other surrounding laryngeal muscles. An example of a conventional ET tube 1, including conventional wire electrodes 3, is shown in FIG. 1. As can be seen there, the ET tube 1 is circular, while the glottic opening 2 is generally triangular, which results in a mismatch between the ET tube and the laryngeal anatomical geometrics, and subsequently contact with the target laryngeal muscles. There have been attempts to improve electrode contact by simply increasing the outer diameter of standard ET tubes to press the electrode on the target laryngeal muscles, which may be the vocal cords. These attempts, however, can lead to difficult and traumatic intubations, as well as the possibility of pressure-induced vocal cord injury, particularly during prolonged operations such as removal of skull base tumors.
Third, there can be anatomic variances in the pharynx and larynx that can force the ET tube to enter the glottis at an angle that reduces contact at the EVC interface, that is, the ET tube may be placed too anterior or too posterior to the laryngeal muscles. An example of the ET tube 1 being placed too anterior (see arrow) to the posterior criciarytenoid muscles 4 so that the electrodes 3 do not have adequate contact with these target muscles 4 is illustrated in FIG. 2. Further, The ET tube may be inserted too deeply or too shallow, which can result in the electrodes being placed inferior or superior to the laryngeal muscles.
Fourth, inadvertent rotation of the ET tube about its longitudinal axis can skew the electrodes away from the target laryngeal muscles and minimize or eliminate proper contact. For example, as shown in FIG. 1, the electrodes 3 have been inadvertently placed opposite the target laryngeal muscles 4, thereby eliminating contact with those target muscles 4. Without contact between the electrodes and the vocal cords, the device may provide a “false negative error”—that is, the device might not emit an alarm indicating detection of the electrical impulse in the muscle. Thus, the surgeon may not appreciate the proximity of the RLN to the electrical probe. Rotation issues may also be exacerbated by a recent shift toward the use of a more rigid, reinforced ET tube (intended to make intubation easier). With this construction, minor rotation of the ET tube at the mouth can result in rotation at the vocal cords or generally within the laryngeal space.
Fifth, to compensate for inaccurate ET tube insertion depth, some ET tubes have increased the un-insulated contact area of the electrodes. This modification, however, can increase the possibility of a “false positive error.” For example, increased exposure of the tube's electrodes can detect inferior constrictor muscle activity. This inadvertently detected stimulation of the inferior constrictor muscle may be misinterpreted as vocal cord stimulation and proximity to the RLN by the electrical probe. Such false positive errors can lead to considerable anatomic disorientation of the surgeon.
Sixth, the EVC contact interface can dry over prolonged periods of contact. This drying can increase impedance which can reduce the detection of the EMG response. In a similar manner, too much moisture from secretions or intentionally applied lubricating jelly may cause shunting of the electrical response away from the electrodes, thereby reducing EVC contact.
Seventh, both false positive and false negative errors can be caused by improperly set coding parameters between the electrodes and the alarm monitor. For example, if the stimulus filter (Ignore Period) is set too long by a surgeon, it may filter out both the true response as well as the stimulus artifact.
Accordingly, there remains room for improving nerve monitoring devices to ensure that the monitored nerves are not damaged or impaired due to inadvertent contact or severing.